A supercritical fluid is widely used in chromatography or specific constituent fractionation because of its permeability to an object, high extractability in mild conditions, and other properties.
In particular, supercritical fluid chromatography is known to have a greater degree of freedom than liquid chromatography in terms of various liquid delivery conditions, such as the flow rate, pressure, temperature, modifier composition ratio.
However, a supercritical fluid has characteristics that are not seen in typical liquids in that a high pressure is required to maintain supercritical conditions and the density of the supercritical fluid greatly varies with pressure. When the density of a supercritical fluid as a mobile phase varies, the elution power of the fluid varies. Therefore, to obtain a satisfactory chromatogram, it is essential to reduce the variation in the density of the supercritical fluid and thus the pressure in a supercritical fluid system. To this end, a special pressure control apparatus is required.
For example, Japanese Patent Publication No. 8-30989 discloses a pressure control apparatus of related art preferably used with a supercritical fluid.
The pressure control apparatus disclosed in Japanese Patent Publication No. 8-30989 has a valve comprising a valve chamber provided in a midway of a channel through which a supercritical fluid flows and a valve element disposed in the valve chamber in such a way that the valve element can travels forward and backward. The valve is used to open and close the channel, through which the supercritical fluid flows, and a control circuit is used to control the pressure in the channel by adjusting the open/close cycle of the valve and the duty ratio thereof.
The apparatus of the related art can reduce the amount of fluid left in the valve chamber and prevent substances contained in the fluid from attaching to the inner wall of the valve chamber.